1. Field of the Invention
The invention relates to a fuel injection system having a booster unit for use with in an internal combustion engine, such as a diesel engine.
2. Description of the Related Art
Fuel injection systems of a boost type using a common rail system in a diesel engine are generally known. In a fuel injection system of this type, high pressure fuel is used as working fluid fed from the common rail to move a booster piston. The booster piston is provided between a pressure chamber and a backpressure chamber in an injector. The booster piston is moved in accordance with a differential pressure occurring between the pressure chamber and the backpressure chamber when the fuel in the backpressure chamber is discharged. The fuel intensified by the booster piston is transferred to a needle valve mechanism of a nozzle portion of the injector.
A fuel injection system is disclosed in, for example, a Japanese patent document (PCT. National Publication No. 2002-539372). The fuel injection system disclosed therein includes a needle valve drive unit that actuates a needle valve of the needle valve mechanism. The needle valve drive unit includes a pressurization chamber that admits fuel fed from a common rail; an open/close valve capable of discharging fuel in the pressurization chamber; and a pressure-receiving piston accommodated in the pressurization chamber. The needle valve drive unit opens the needle valve in conjunction with the discharge of fuel being preserved in the pressurization chamber.
In fuel injection systems of the type described in the patent document, a large amount of fuel is used to operate the booster unit. As such, in comparison to a fuel injection system without a booster unit, a common rail with a large capacity is necessary depending on the type.
The common rail pressure is controlled to an optimal value through, for example, adjustment of the amount of feed from a supply pump in correspondence to the engine operation mode. For example, in the event that the engine is running on high load and high speed, the supply pump is controlled to cause the common rail pressure to be higher than in the event that the engine is running on low load and low speed.
Under these circumstances, demand arises in that the common rail pressure is controlled to lower in a short time in an engine transient state, such as in the event that the engine load state transitions from a high load state to a low load state, and an accelerator opening (i.e., the amount of acceleration pedal operation) transitions from a large state to a small state. However, in a common rail having a relatively large capacity suited to a booster unit, it takes a time for pressure reduction, thereby causing a response delay. As such, there can occur high pressure injection of fuel in a low load state, wherein it is contemplated that inverse effects are imposed on exhaust gas, etc.